The invention pertains to a method for optically measuring the weld penetration depth, particularly in welding, drilling or machining processes carried out by means of laser beams.
In order to optically measure the weld penetration depth, it is known to utilize optical distance measurement sensors that operate according to the principle of optical short-coherence interferometry, in which the measurement light is split into a measurement light beam, which is also simply referred to as measurement beam below, and a reference light beam, which is also simply referred to as reference beam below. The measurement and reference light beams reflected from a measurement arm and a reference arm are superimposed with one another in order to determine the desired distance information from the path differences between the measurement arm and the reference arm.
In this case, the field of application covers machining processes that require the precise and automated positioning of the measurement light beam at a position in the region of the interaction zone between the working laser beam and the workpiece, particularly at the vapor capillary or so-called keyhole produced by the working laser beam in its point of incidence, e.g. in laser welding processes with in-line monitoring of the weld penetration depth to be controlled.
Known technical solutions for precisely positioning the optical measurement light beam in laser welding processes utilize camera-based methods for determining the measurement beam position relative to the working laser beam. These methods are based on an indirect determination of the position of the measurement light beam on the workpiece surface, which is required for measuring the weld penetration depth.
However, the optimal position of the measurement light beam relative to the processing beam for a reliable measurement of the weld penetration depth is dependent on different process parameters—such as the advance speed and the material of the weld metal—and therefore cannot be determined with sufficient accuracy by means of indirect position determination methods.
DE 101 55 203 A1 describes a laser machining device with an observation unit that is realized in the form of a short-coherence interferometer for acquiring surface measurement data. For example, the depth of focus can also be monitored and controlled with a measurement at the machining point. However, it is not described how the measuring point, i.e. the point of incidence of the measurement beam, has to be aligned relative to the machining point in order to obtain a reliable and accurate measurement of the depth of focus or keyhole depth.
DE 10 2015 012 565 B3 concerns a device and a method for increasing the accuracy of an OCT measurement system for laser material machining and describes the positioning of a measurement beam generated by an optical coherence tomograph relative to the position of the laser beam during the machining process with the aid of a spatially resolving sensor such as a camera. In this case, a relative offset between the processing beam and the measurement beam is determined from the spatially resolved information provided by the sensor with consideration of a measurement beam position on a workpiece. However, the positioning of the measurement beam relative to the vapor capillary, i.e. relative to the keyhole, is not described.
DE 10 2013 015 656 B4 concerns a method for measuring the weld penetration depth, in which two measurement beams are guided through processing optics. A first measurement beam is directed at the base of the keyhole in order to measure the distance from the keyhole bottom and a second measurement beam is directed at the surface of the component in order to measure the distance from the component surface. The weld penetration depth can be determined from these two distances. However, it is not described how the measurement beam is aligned at the position of the keyhole.
As described above, all known methods are based on an indirect determination of the position of the measurement beam on the workpiece in order to measure the weld penetration depth. However, this does not make it possible to determine the exact position relative to the vapor capillary, i.e. relative to the keyhole, because it is difficult to measure the exact position of the keyhole in the processing region, i.e. in the region of incidence of the working laser beam, with imaging methods.